1. Field of the Invention
The present invention generally relates to developer quality monitoring devices and more particularly to a developer quality monitoring device which can monitor deterioration in the state of a developer consisting of carriers admixed with toners, the state being in particular electrostatic adherence of the toners to the carriers.
2. Description of the Prior Art
In electrophotographic copying apparatus and electrostatic recording apparatus, an electrostatic latent image formed on a recording medium is developed with a developer into a visual image. In many applications, the developer used for the development is a mixture of resin toner particles and carriers such as iron powders, that is, a binary component developer, and the toner is charged so as to be adhered to the surface of the carrier by an electrostatic force. Since the carrier is typically comprised of a magnetic material, the developer can be attracted by a magnet to form a magnetic brush. When the magnetic brush slightly touches the latent image surface and softy wipes the same through electric static force, the toner is deposited onto the recording medium by an electrostatic force to thereby develop the latent image. The developer for use in such a development process, however, deteriorates in its characteristics under the influence of humidity or after a long-time usage which is comparable to its life. The degraded characteristics raise such problems as the density of developed images is decreased even when the mixing ratio between toner and carrier or the toner density is constant, the recording medium texture is contaminated, and the development unit or peripheral equipments are contaminated. Accordingly, it is necessary to monitor the decay condition of the developer.
An approach has hitherto been made which measures the toner density, developed image density or contaminants on the texture to detect the decay condition of the developer, as seen from Japanese Patent Kokai (Laid Open) Nos. 50750/75, 29725/78, 49438/78, 49439/78, etc. This conventional measure, however, requires a reference area on the recording medium and the formation of a latent image as a criterion on the reference area. Consequently, results of the detection are affected by conditions for the formation of criterion latent image and correct detection of the deteriorated state of the developer per se is impossible.
Apart from the above disadvantages attributable to deterioration of the developer, it is experimentally proven that the amount of toners which are electrostatically adhered to carriers is decreased as the developer becomes decayed or deteriorated. Assumptively, when the developer greatly absorbs humidity and when the developer is used for its life so that scrap toner is permanently adhered by a physical force to the surface of the carrier, the toner cannot be charged sufficiently by stirring the mixture with the result that the electrostatic adherence of the toner to the carrier is degraded. If the insufficient charging of the toner is aggravated under the condition that the toner density is constant, the number of free toners (which cannot be adhered to carriers properly) is increased and the free toners aggravate contaminants on the recording medium texture and contamination of the equipments. On the other hand, the number of toners to be charged properly and adhered to carriers is so reduced as to decrease density of the developed image.